Every ecchi has a plot. What these critics probably mean is that ecchi is characterized by what they consider to be poor plots, not an absence of plot.

For instance, they might mean that the characters of the series in question are shallow or recycled, that the sequencing felt like it never culminated in a clear direction for the story, that the story did follow a clear direction but its path was cliche, that the characters' motivations to follow the plot were dubious, or any number of other things.

What they cannot possibly mean is that ecchi series generally have literally no plot whatsoever. That claim would be ludicrous. If two events are connected in any sort of coherent fashion, you've got a plot. You don't even need dialogue, just connect two events in some way that makes any kind of sense whatsoever. "Plot" really means nothing more than connection of two or more events into some sort of cohesive story. For example:

Sally bought some eggs.
Sally cooked an omelette with the eggs she bought.

BOOM. Plot. One character, two events with a discernible temporal order and a clear connection between them. A plot can be that shallow and still count as such. Sally has no last name, no appearance, she doesn't have any dialogue, we don't even really have a setting. This is a very crappy plot. But it is a plot.

It depends on the show. I've seen plenty of anime that have absolutely no plot and many of which are not ecchi. Given that some focus too much on the ecchi and it takes away from the plot, but if it is used well then it can flow with the show and just seem natural.

Ecchi is such a broad definition that one can argue for ages as to what does and does not fit under that umbrella. ... That being said, ecchi elements do not bother me. I have been watching anime for a long time.

Saying that ecchi has no plot, is like saying that electronic music is just beats beats beats.
It's something that closed minded people say.

Of course ecchi has plot.
Of course electronic musis is more than just beats beats beats.

Example:
Air Gear - had a pretty cool plot imo. I watched it for the plot only back when I watched it, thus why I want a second season.
Haganai - has a plot.
Elfen Lied - had a good plot.
Freezing - got a plot after like 7 episodes, which is why I bothered to continue :P
Fight Ippatsu! Juuden-Chan!! - mostly ecchi, but it had a plot. The plot is why I want a second season, but that'll probably never happen. Stupid cliffhanger it ended on.
Hatsukoi Limited - got plot. Good romance imo.
Highschool of the Dead - I watched it for the plot. Ok not quite true, I heard it was about zombies and ecchi, so I became very interested in it But I continued watching it becaus of the plot. And I want more, which I'm getting :D
Sankarea - got a pretty cool plot.
Sekirei - I enjoyed the plot, which is why I want a third season.
Zero no Tsukaima - watched it for the plot. Haven't completed last season though, it was shit.